This invention pertains to digital imaging. More particularly, this invention relates to methods and apparatus for reducing colorant used in digital printing.
A digital printer receives image data from a computer and places colorant, such as ink or toner, on a sheet of receiving material, such as paper or transparency plastic. The printer may use a variety of different technologies to transfer colorant to the page, including inkjet, thermal dye transfer, thermal wax, electrophotographic, silver halide, and other processes. High quality digital color printers typically use a combination of cyan, magenta, yellow, and black (“CMYK”) colorants, although some printers may use more than or less than these four colorants. Each individual colorant commonly is referred to as a “channel,” with the amount of colorant for each channel typically specified as a percentage between 0 and 100%. Thus, on a four-color printer, the maximum amount of colorant that may be specified is 400%, corresponding to 100% on all four channels.
If excessive colorant is used, however, undesirable image artifacts such as bleeding (an undesirable mixing of colorants along a boundary between printed areas of different colorants), cockling (warping or deformation of the receiving material that may occur from using excessive colorant), flaking and smearing may result that produce an unacceptable print. In severe cases, excessive ink may cause the receiving material to warp so much that it interferes with the mechanical operation of the printer and may damage the printer. In addition, for many color printers, satisfactory density and color reproduction can generally be achieved without using the maximum amount of colorant. Therefore, using excessive colorant not only may cause undesirable image artifacts and may damage a printer, but it also wastes colorant. Generally, the amount of colorant needed to cause undesirable image artifacts (and therefore be considered excessive) depends on the receiver material, colorant and printer technology.
To minimize the effects of excessive colorant, previously known printing systems often include colorant limitation devices or methods. Referring to FIG. 1, a conventional colorant limitation system is described. Colorant limitation system 10 includes colorant limiter 12, which receives input data and provides colorant-limited input data to print output device 14, which generates a print document in accordance with the colorant-limited input data. Previously known techniques for limiting colorant usage, such as the system of FIG. 1, have typically performed colorant reduction without including the effects of actual print output device performance. For example, Allen et al. U.S. Pat. No. 5,633,662 (“Allen”) describes a process for controlling ink volume in liquid ink printing systems by comparing the total specified ink volume per pixel to a selected maximum total ink volume per pixel. Depleted ink volumes are formed by applying a scaling factor to each pixel, or to those pixels having ink volumes that exceed a threshold ink volume. Likewise, Li et al. U.S. Pat. No. 5,872,896 (“Li”) describes an ink limiting algorithm in which pixels that exceed a total ink limit are reduced, with some pixels reduced to values that are significantly below the total ink limit. Neither Allen nor Li, however, describe ink limiting methods or apparatus that include a direct measurement of actual printer behavior or specified behavior as part of the ink reduction determination.
Indeed, previously known colorant-reduction techniques typically are optimized for speed, and do not consider variations in engine state that may significantly affect print device output. For example, a print output device may include one or more color channels, with each color channel having a nominal output response. As a result of ageing or device imperfections, one or more color channels may have an actual output response that differs from the nominal response. For example, the output of one color channel may be only 50% of the nominal output. As a result, although the specified amount of colorant for that channel may be 60%, the actual amount of colorant provided by the channel may be only 30%. Nevertheless, a conventional ink limitation algorithm would operate in the usual manner, although the print device actually can tolerate a higher specified amount of colorant because the actual output has already been limited by some other mechanism or defect.
In view of the foregoing, it would be desirable to provide methods and apparatus for performing colorant limitation that prevent unnecessary colorant limitation.
It further would be desirable to provide methods and apparatus for performing colorant limitation that include the effects of actual print device performance.
It also would be desirable to provide methods and apparatus for performing colorant limitation that may be modified to accommodate changes in print output device performance.
It additionally would be desirable to provide methods and apparatus for performing colorant limitation that include the effects of specified device performance.